This chapter focuses on advancements in multi-carrier generation and large capacity optical transmission systems using recirculating frequency shifter (RFS) technology. Firstly, a single-sideband (SSB) modulator-based RFS is employed to generate 50 frequency-locked, 12.5-GHz-spaced optical carriers with exceptional flatness and stability. Critical factors affecting carrier flatness are experimentally analyzed. Secondly, a novel dual-band RFS architecture based on a dual-parallel Mach–Zehnder modulator (DP-MZM) is proposed, doubling efficiency while halving cost. Simulations demonstrate the generation of carriers with flatness as low as 0.6 dB and carrier-to-noise ratios (CNR) exceeding 40 dB under optimized RF drive conditions. Thirdly, a 2.56-Tb/s polarization-division-multiplexed return-to-zero 16QAM (PDM-RZ-16QAM) coherent optical WDM (CO-WDM) system is experimentally validated over 800 km of standard single-mode fiber, achieving a spectral efficiency of 5.1 b/s/Hz.